Hitherto, hydrocarbylaluminoxanes were prepared by careful addition of water, at low temperatures, as a reagent to a solution of trialkylaluminum. Such processes have been the subject of several patents; for example, U.S. Pat. No. 3,300,458 to Manyik etal., U.S. Pat. No. 4,722,736 to Edwards et al., U.S. Pat. No. 4,730,071 to Schoenthal et al., and U.S. Pat. No. 4,908,463 to Bottelberghe.
Several alkylaluminoxanes including mixtures of aliminoxanes, have been employed as co-catalysts in olefin polymerization. However, the most useful and widely used co-catalyst, especially in single-site or metallocene-based olefin polymerization is methylaluminoxane (MAO). Despite the importance of methylaluminoxane as co-catalyst, structural characterization has largely been hampered by limited solubility and solution instability. In solution, methylaluminoxane is believed to undergo rapid and significant dynamic equilibrium between several aluminoxane species including residual trimethylaluminum (TMA). For a relatively recent review of several aspects of structural advances involving methylaluminoxane and aluminoxanes in general, see Pasynkiewicz, Polyhedron, 1990, 9, 429.
T. Mole and co-workers have described the chemistry of exhaustive methylation of alcohols, ketones, and carboxylic acids by treatment with trimethylaluminum as early as 1974. See Aust. J. Chem., 1974, 27, 1665. Although indicating that the by-product of these reactions was an aluminoxane, these MAO by-products were destroyed by hydrolysis, because those authors were more interested in the methylated hydrocarbons produced in the reactions. A recent review by J. J. Eisch, Comprehensive Organometallic Chemistry II, Vol. 1, 1995, 452, also confirmed that the exhaustive C-methylation chemistry described by T. Mole and co-workers must have been accompanied by aluminoxane formation. Eisch further added that such compositions are not suitable for catalytic olefin polymerization because of the presence of potentially deactivating Lewis base in the reaction product (see equation 59 of this Eisch review).
In published patent application WO 97/14699 (Apr. 24, 1997), G. M. Smith et al. describe formation of modified polyalkylaluminoxane compositions by initially treating a composition comprising trialkylaluminum with a reagent containing a carbon-oxygen double bond, followed by hydrolysis of the resulting composition. Reagents with a carbon-oxygen double bond referred to are carbon dioxide, a ketone, an aldehyde, a carboxylic acid, a carboxylic acid ester, a carboxylic acid anhydride, and a carboxylic acid amide. The products are indicated to contain oligomeric alkylaluminoxane and moieties having the structure --OC(R).sub.3, where R is hydrocarbyl, such as lower alkyl, such as methyl.
More recently, in published patent application, WO 97/23288 (Jul. 3, 1997) G. M. Smith et al. describe a non-hydrolytic (without water) preparation of polyalkylaluminoxane compositions by essentially using the method of Mole et al. Thus trimethylaluminum is treated with certain oxygen-containing organic compounds such as an alcohol, a ketone, or a carboxylic acid to form aluminoxane and C-methylated hydrocarbons. Reaction with carbon dioxide is also referred to in this Smith et al. application. However, unlike Mole et al., the methylaluminoxane composition product was not destroyed by hydrolysis.
The latter published patent application by Smith et al. uses similar conditions to those of Mole et al. characterized by long reaction times and high temperatures. Several publications have described the enhanced acceleration of gel formation process in methylaluminoxane solution by prolonged heating, especially at high temperature; including, for example, U.S. Pat. No. 5,329,032 to N. H. Tran et al. and Japanese Patent Publication No. 49293/92.
While it is conceivable that viable preparation of MAO using the method of Mole et al. is possible, it would be highly desirable to find an MAO production process capable of using milder conditions in order to ensure the integrity of the resulting methylaluminoxane composition as hydrocarbon-soluble and storage-stable material. Commercial methylaluminoxane products are often transported to distant overseas destinations requiring several weeks of transit. It is therefore important to avoid gel formation or excessive viscosity increases that could hamper removal or transfer of the aluminoxane products from tank to tank. Furthermore, transfer of methylaluminoxane solution from a storage tank to polymerization reactor is facilitated by the absence of gels, solid-forming precipitates, or high viscosity liquids due to storage instability of the solution.